There are a number of different types of devices and methods for storage of data or information that do not lose the data or information thereon. Such storage devices or mediums are sometimes referred to as non-volatile types of memory or storage devices and include magnetic storage mediums (e.g., magnetic disc or hard drives, magnetic tape drives, flash memory cards, EEPROMS) and magneto-optical storage mediums (e.g., CD, CDROM, CDRAM, DVD, DVD±R or DVD±RW). Conventional random access memory (e.g., SRAM, DRAM) on the other hand loses any data or information contained in the memory when power is lost to the device that uses the memory. Thus, conventional computers and processing systems typical include different types of memory systems that use different types of memory so as to meet operational needs or requirements (e.g., RAM for use with the CPU and a non-volatile memory-magnetic hard drive for long term data storage).
Notwithstanding the efficacy of conventional long term magnetic storage devices, new types of patterned media for information storage have been explored to replace the continuous magnetic films used in current magnetic storage in order to achieve higher storage density. M. Helm, K. Ounadjela, J. P. Bucher, F. Rousseaux, D. Decanini, B. Bartenlian, C.; Chappert, Science 1996, 272, 1782; K. J. Kirk, J. N. Chapman, S. McVitie, P. R. Aitchison, C. D. W. Wilkinson, Appl. Phys. Lett. 1999, 75, 3683; M. Herrmann, S. McVitie, J. N. Chapman, J. Appl. Phys. 2000, 87, 2994; C. A. Ross, Annu. Rev. Mater. Res. 2001, 31, 203. In patterned media, a bit, either “0” or “1”, is stored in a single magnetic entity, rather than in a bit consisting of a number of grains as in continuous film. Such new types of patterned media for information storage include magnetic dots and magnetic disks. Both small dots and discs with sizes less than about 200 nm, however, have been found to have poor magnetic stability especially when the separations between the small entities are small.
Nanoscale magnetic entities play a prominent role in many devices in which the shape and dimension of the entities dictate their intricate magnetization configurations and switching characteristics. M. Helm, K. Ounadjela, J. P. Bucher, F. Rousscaux, D. Decanini, B. Bartenlian, C. Chappert, Science 1996, 272, 1782; K. J. Kirk, J. N. Chapman, S. McVitie, P. R. Aitchison, C. D. W. Wilkinson, Appl. Phys. Lett. 1999, 75, 3683; M. Herrmann, S. McVitie, J. N. Chapman, J. Appl. Phys. 2000, 87, 2994; A. Ross, Annu. Rev. Mater. Res. 2001, 31, 203. Of particular interest are small circular disks that can acquire the so-called vortex state, in which the magnetic flux is confined within the magnetic entity and creates no stray fields, so that the cross-talk between entities can be reduced. However, the vortex core (with magnetization pointing out of the plane of the disk) tends to destabilize the vortex state, which consequently is replaced by the single-domain state in sufficiently small disks. In supermalloy disks, the transition from vortex state to single-domain state occurs at a diameter of a few hundred nanometers. However, the instability of the vortex core in a small circular disk can be circumvented by removing the central region of the disk to form a ring structure, the so-called “nanorings”. Nanorings possess highly stable vortex states in which the magnetic moments form circular loops along the circumference. The chirality of the vortex states, clockwise and counterclockwise, can be utilized to store information of “0” and “1”. For nanorings with 100 nm to 500 nm in diameter, there are two processes for switching their magnetizations: vortex formation process and onion rotation process.
Experimental results for micron-diameter rings and 300-800 nm diameter octagonal ring structures show the existence of just two different magnetic states: one being the flux-closure or vortex state and the other a bi-domain state with two 180 degree domain walls, called an onion state. There also has been reported in US2004/0211996 another metastable state called a twisted state for smaller diameter rings. This twisted state contains a 360 degree domain wall and can exist over a wide range of applied fields. To attain such a twisted state, the nanoring is configured so as to include a deviation (e.g., notch), artifact so that the domain wall is pinned at the locations of the deviations. A nanoring having such a deviation or configuration was referred to as being asymmetric. These nanorings also were fabricated by a liftoff process from ring-shaped patterns written into a resist layer by electron-beam lithography.
In such structures, each ring can store a bit of information depending on its magnetic state. The rings are written by applying magnetic fields (the fields are produced by passing currents through adjacent conductive lines). The data-bit in the rings is read back by detecting the rings' electrical resistance, which depends on their magnetic states. The dependence of resistance on magnetic state is called magnetoresistance. To use magnetoresistance for data readback it is most convenient to make the memory element out of a magnetic multilayer, for instance two magnetic layers separated by a non-magnetic spacer. In such a multilayer (called a spin-valve or tunnel junction), the resistance can vary by up to about 10-50% depending on the relative magnetization directions of the two magnetic layers and the structure of the multilayer.
In sum, small magnetic nanorings can not only maintain stable vortex states, but also hold the potential for information storage in the two chiralities of the circulating magnetization. These properties of nanorings have also led to the proposal of high-density, vertical, magnetic, random access memory (VMRAM) consisting of multilayered nanorings with exceptional stability and desirable switching characteristics. J. G. Zhu, Y. Zheng, G. A. Prinz, J. Appl. Phys. 2000, 87, 6668.
For these reasons, the study of magnetic nanorings of various sizes has been actively pursued recently. Electron-beam lithography (EBL) is one method used for fabricating magnetic nanorings. Most of the magnetic rings previously reported have been in the micrometer-size range, with few reports existing on sub-micrometer-sized nanorings. The arrays of magnetic rings made by EBL usually have a small number of rings (e.g., less than 103) with low areal densities (e.g., 0.5 rings μm2 or less). As such, the magnetic signal of the magnetic nanorings has been too weak for full characterization using magnetometry. Instead, the magnetic characteristics have been measured or inferred by surface magneto-optic Kerr effect (MOKE) measurements, resistance measurements, Hall-sensor measurements, and magnetic force microscopy. In these cases, the external applied magnetic field could only be applied in certain directions, and not all, so as not to interfere with the specific measuring technique used. Also, magnetic nanorings fabricated by e-beam lithography are limited to a small area with low areal density.
The present inventors have discovered from magnetic measurements of symmetrically shaped nanorings, that when the diameters of symmetrically shaped magnetic nanorings are of the order of 100 nm, the magnetic reversal undergoes two processes with similar probabilities. One is the vortex reversal process, in which the vortex states can be sustained. The other one is the rotating onion process that involves no vortex states.
It thus would be desirable to provide a ferromagnetic nanoring including an assemblage of such nanorings having small diameters and methods for making such ferromagnetic nanorings. It would be particularly desirable to provide such methods for fabricating such ferromagnetic nanorings having controlled dimensions including diameter, width, and thickness as well as areal density. It also would be desirable to provide such methods for fabricating nanorings that yield nanorings that are symmetrically or asymmetrically shaped. It also would be desirable to provide methods for controlling application of the magnetic field to the nanoring(s), in particular the asymmetrically shaped nanorings so as to thereby control the formation of a vortex state in such nanorings.